Much of the information we collect has a half life, or situations change and that information, while still valid, is not applicable to the issues we are facing currently. The vast majority of things we learn end up getting discarded at some point.
Think of the most recent book you read. How many of the details actually stuck with you at the moment you closed it. Now think of a book you read 10 years ago. How much of that do you remember?
I took years of mathematics and was able to do well on tests, but 40 years later I only recall the "shape" of PDE solutions and couldn't actually solve anything anymore. Instead my brain is stuffed full of arcane knowledge I need to do my job, and if I don't use it within a year, I probably need to learn it all over again.
This is a valid question for different reasons, too. I have a problem with the speech and behavior of immortal characters in books - why would a 300 year old powerful vampire behave like an immature emo teenager?
There's a reason we associate wisdom with age. The more times anyone of reasonable intelligence makes mistakes, the more opportunities they have to learn, and their behavior changes according to the degree to which they take on the lessons of life.
One big danger of immortal dictators is the simple fact that they'll stop learning. Through wealth and power they shield themselves from the consequences of mistakes, getting themselves and their people stuck in a local minima.
Imagine immortal Mitch McConnell, ever increasingly wealthy through passive income, maintaining power and privilege for his constituents and thus his hold on a senate seat.
If life extension pans out, liberal societies will have to impose term limits in a serious and well considered way. Humans aren't ready for the existing pace of technological development, and we're going to encounter an exponentially increasing number of problems, like the politics of immortality. The best thing we could do would be to maximize freedom of expression and minimize the duration of social institutions to achieve sufficient maneuverability to adapt to modern life.
It wouldn't be a limitation for centuries, or possibly millenia. The brain very efficiently packs information, and integrates with external storage. We'll be able to digitally augment brains directly long before temporal memory capacity becomes a problem.
